A. Field of the Invention
This invention relates to an apparatus and method for controlling a well during drilling and more particularly is directed to an apparatus and method for controlling formation conditions encountered during drilling operations.
B. The Prior Art
Occasionally during drilling operations the well is drilled into a formation having an abnormally high gas pressure. Either a gas formation or a gas-liquid formation may be encountered. Such formations may produce blowout conditions, and, unless quickly remedied, the well can get out of control causing a loss of well fluids and destruction of drilling equipment.
Conventional drilling equipment includes a plurality of blowout preventors in a blowout preventor stack. However, surface blowout preventors do not control a well at the source of the problem, namely downhole at the high pressure formation region. Surface blowout preventors can only attempt to confine the high pressure within the well. They are not entirely successful. When a gas bubble makes its way up through the annulus between the drill string and the well, the well may be in danger. A gas bubble high in the annulus means that the hydrostatic head of drilling fluid has become ineffective and the surface casing and equipment may not be able to withstand the high pressure gas to confine the same. Additionally, the gas bubble itself may deteriorate the rams of the blowout preventor to an extent which renders them ineffective.
Attempts have been made to provide a downhole blowout preventor and a method of controlling a well utilizing the same. These attempts have produced systems which still have disadvantages.
U.S. Pat. Nos. 3,283,823 and 3,322,215 to Warrington disclose, respectively, an apparatus and method for controlling downhole formation pressures. These patents disclose utilizing an open bore packer located in the drill string immediately above the drill bit to close the annulus of the well. Another closure means is provided to close the drill string bore at the packer. Communication is then provided from the drill string bore above the packer to the annulus above the packer. Since the hydrostatic head of pressure provided by circulating drilling fluid is ineffective below the packer, the packer must be located directly above the drill bit. Locating the packer directly above the drill bit means that the packer must seal the open bore of the well. In soft sedimentary formations sealing the open bore of the well is difficult and may be impossible.
U.S. Pat. No. 3,427,651 to Bielstein et al also discloses utilizing a packer positioned in the drill string immediately above the drill bit to close the annulus of the well. A communicating means to permit drilling fluid to circulate from the bore of the drill string to the annulus above the packer is provided, although the bore remains open. Such a device still has the disadvantage that it requires the use of an open bore packer.
U.S. Pat. No. 3,503,445 to Cochrum et al also discloses the utilization of an open bore packer to seal the annulus between the drill string and the well wall. A control plug, which is transmitted downward through the drill string, shifts a sleeve valve so that the packer may be inflated and so that communication may be established between the drill string bore and the annulus. The plug also closes the bore. Again, the disclosed system contains the two disadvantages of requiring an open bore packer which may not be able to seal the annulus in a soft formation and of preventing continued circulation of drilling fluid below the packer.
U.S. Pat. No. 3,710,862 to Young et al does disclose a packer and crossover valve combination utilized for completing a well. The patent discloses circulating fluid down an operating string above the packer, through a service seal unit, and out into the annulus below the packer. This operation is carried out after the well has been drilled and the drill string removed. Fluids may then continue to circulate by flowing from a point below the packer up through the annulus of the service seal unit and out into the annulus around the operating string above the packer. Such a packer and crossover combination is not concerned with controlling high formation pressures that may be encountered during drilling operations and, indeed, is operated to stimulate wells having low formation pressures.
U.S. Pat. No. 3,527,296 to Malone teaches packing off a drill string in the cased area of a well, but does not provide for circulation to permit treatment of a well.
While the prior art has recognized that the entire well should be treated (the open hole packers are positioned close to the drill bit) they do not teach a system for treating the entire column of mud in a well in which an open hole packer is ineffective.